Friday, November 30, 2007

1.9 What is a diopter?

Since there have been references to refractive power in the previous posts, a quick clarification is in order:

The diopter (D) is the basic measurement unit of optics; one diopter is equivalent to 1-meter focal length. The calculation of lens focusing power D is:

D = 1/focal length (in meters).

The higher the dioptric value, the stronger the lens focusing power; for example, a 10D lens will have a focal length of 0.1m or 10 times shorter than that of a 1D lens.

A plus (convex) lens has a real focal point (parallel light rays get converged onto one point) whereas a negative (concave) lens, a virtual focal point.

In the eye, the corneal power is about 44D (taking into account the refractive index of air=1 and that of the aqueous humor=1.33), and the power of the crystalline lens about 16D - for a total of 60D. This is essentially a 15X magnifier (60D/4 = 15X assuming 25cm of viewing distance).

Incidentally, if the crystalline lens is removed as in cataract surgery, then all 16D must be restored using spectacles, contacts, or more frequently intraocular lens implants (IOLs).

Only in lay terms, for example, -1.25D = 125 degrees of myopia; although there is really no such thing as degrees of power in optics.